Several safety devices are disposed in an automobile to secure occupant safety. An air bag device in which an air bag is deployed by ejecting gas generated by an inflator during an emergency such as a collision serves as one of these safety devices.
In a technique disclosed in relation to this type of air bag device, a folded air bag is stored in an air bag case such that the air bag is covered by a deployment restriction sheet from a vehicle body rear side to a vehicle body front side (Patent Document 1, for example). With this technique, a rapidly deploying air bag is prevented from interfering with an occupant during an early stage of deployment.    Patent Document 1: Japanese Unexamined Patent Application Publication 2001-334900
The vehicle body rear side of the deployment restriction sheet according to Patent Document 1 is fixed, but the vehicle body front side is not fixed and therefore forms a free end.
With the deployment restriction sheet according to Patent Document 1, a position of a folded back portion may shift during travel after the air bag device has been attached to the vehicle body. When the position of the folded back portion of the deployment restriction sheet shifts, the air bag may deploy before the deployment restriction sheet is fully extended, leading to variation in deployment behavior. When the deployment behavior of the air bag varies, a cushioning action cannot be applied to the occupant effectively.
As shown in FIG. 8, to prevent the position of the folded back portion of the deployment restriction sheet from shifting, a deployment restriction sheet 2 is covered from above by a flap 5 having two fixed ends. However, 100% reliability cannot be achieved with this method either.
In FIG. 8, reference number 1 denotes an air bag stored in an air bag case 3 in a folded state, and 4 denotes a lid that is ruptured when the air bag 1 expands. Further, reference number 6 denotes a retainer for fixing an opening portion of the peripheral edge of the air bag 1 to the air bag case 3. The retainer 6 fixes a second end of the deployment restriction sheet 2 on the vehicle body rear side to a second end of the flap 5 using an attachment bolt 6a and a nut 8 fastened thereto.
A problem to be solved by the present invention is that when a folded air bag is covered by a deployment restriction sheet from a vehicle body rear side to a vehicle body front side but only the vehicle body rear side of the sheet is fixed, the position of a folded back portion may shift. A further problem is that even when the deployment restriction sheet is covered by a flap, positional deviation of the folded back portion cannot be prevented with 100% reliability.
In consideration of these points, an air bag device according to the present invention solves the problems described above by fixing both ends of the deployment restriction sheet, providing a rupturable portion on the vehicle body front side of the deployment restriction sheet, and joining the folded back portion of the deployment restriction sheet such that a joint portion separates before the rupturable portion ruptures.